U.S. patent number 5,846,366 [Application Number 08/698,284] was granted by the patent office on 1998-12-08 for method for interconnecting an electronic device using a transferable solder carrying medium.
This patent grant is currently assigned to Lucent Technologies Inc.. Invention is credited to Sungho Jin, Mark Thomas McCormack.
United States Patent |
5,846,366 |
Jin , et al. |
December 8, 1998 |
Method for interconnecting an electronic device using a
transferable solder carrying medium
Abstract
In accordance with the invention, an electronic device having
one or more contact pads is placed in contact with a carrier sheet
bearing an array of transferable solder particles. Heat is applied
to adhere the solder to the contact pads, and solder is selectively
transferred onto the contact pads. In a preferred embodiment the
solder-carrying medium comprises elastomeric material and the
solder particles comprise solder-coated magnetic particles.
Application of a magnetic field while the elastomer is curing
produces a regular array of solder coated particles. Using this
method, devices having smaller than conventional contact structures
can be readily interconnected.
Inventors: |
Jin; Sungho (Millington,
NJ), McCormack; Mark Thomas (Summit, NJ) |
Assignee: |
Lucent Technologies Inc.
(Murray Hill, NJ)
|
Family
ID: |
22952436 |
Appl.
No.: |
08/698,284 |
Filed: |
August 14, 1996 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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251548 |
May 31, 1994 |
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Current U.S.
Class: |
156/233; 156/234;
156/300; 228/180.22; 228/248.1; 156/235; 156/297; 257/E21.508;
257/E21.511 |
Current CPC
Class: |
H01L
21/4853 (20130101); H01L 21/6835 (20130101); H01L
24/11 (20130101); H05K 3/3485 (20200801); H01L
24/81 (20130101); H01L 2924/01033 (20130101); Y10T
156/1089 (20150115); H01L 2224/11003 (20130101); H01L
2924/01051 (20130101); H01L 2924/01047 (20130101); H05K
2203/0528 (20130101); H01L 2924/01079 (20130101); H01L
2224/05611 (20130101); H01L 2224/05644 (20130101); H01L
2224/16 (20130101); H01L 2224/13099 (20130101); H01L
2924/0105 (20130101); H05K 2201/0133 (20130101); H05K
2201/083 (20130101); H01L 2224/11334 (20130101); Y10T
156/1093 (20150115); H05K 2203/0156 (20130101); H01L
2924/01024 (20130101); H01L 2924/01105 (20130101); H01L
2924/0103 (20130101); H01L 2924/01013 (20130101); H01L
2924/01027 (20130101); H01L 2924/0106 (20130101); H05K
2201/0218 (20130101); H01L 2924/01006 (20130101); H01L
2924/01025 (20130101); H05K 2203/0338 (20130101); H05K
2203/104 (20130101); H01L 2224/1111 (20130101); H01L
2924/01029 (20130101); H01L 2924/014 (20130101); H01L
2224/05147 (20130101); H01L 2224/81801 (20130101); H01L
2924/01322 (20130101); H01L 2924/01005 (20130101); H01L
2924/01082 (20130101); H01L 2924/15787 (20130101); H05K
3/3478 (20130101); H05K 2203/0425 (20130101); H01L
2924/15787 (20130101); H01L 2924/00 (20130101); H01L
2224/05611 (20130101); H01L 2924/00014 (20130101); H01L
2224/05644 (20130101); H01L 2924/00014 (20130101); H01L
2224/05147 (20130101); H01L 2924/00014 (20130101) |
Current International
Class: |
H05K
3/34 (20060101); H01L 21/02 (20060101); H01L
21/48 (20060101); H01L 21/67 (20060101); H01L
21/60 (20060101); H01L 21/68 (20060101); B23K
031/02 () |
Field of
Search: |
;156/230,233,234,235,297,299,300 ;228/180.22,248.1,255 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
H H. Manko Soldering Handbook For Printed Circuits And Surface
Mounting. .
IBM Technical Disclosure Bulletin, vol. 37, No. 4B, Apr. 1994, pp.
117-119. .
IBM Technical Disclosure Bulletin, vol. 19, No. 6, Nov. 1976, p.
2049. .
IBM Technical Disclosure Bulletin, vol. 17, No. 2, Jul. 1974, p.
627..
|
Primary Examiner: Simmons; David A.
Assistant Examiner: Rivard; Paul M.
Attorney, Agent or Firm: Mathews, Collins, Shepherd &
Gould
Parent Case Text
This application is a continuation of application Ser. No.
08/251,548 filed May 31, 1994 now abandoned.
Claims
We claim:
1. A method for connecting an electronic device having one or more
electrical contact pads to a second device comprising the steps
of:
providing said electronic device having one or more contact
pads;
providing a carrier sheet having a plurality of solder balls
transferably disposed thereon, said solder balls randomly
distributed on said carrier sheet and having diameters in the range
0.5 to 50 .mu.m;
adhering said device to solder balls on said carrier sheet to
adhere a plurality of solder balls to each contact pad;
transferring the adhered solder balls to said contact pads; and
placing the solder-carrying pads in contact with said second
device.
2. The method of claim 1 wherein said solder balls are adhered to
said contact pads by applying heat to said particles.
3. The method of claim 1 wherein said carrier sheet is flexible and
said adhered solder is transferred by peeling said carrier sheet
away from said electronic device.
4. The method of claim 1 wherein said carrier sheet comprises a
layer of elastomeric material and said solder balls are partially
embedded in said elastomeric material with protruding portions for
contacting said pads.
5. The method of claim 1 wherein said solder balls comprise
solder-coated magnetic particles.
Description
FIELD OF THE INVENTION
This invention relates to methods for connecting electronic
circuits and devices and, in particular, to such methods using an
array of transferable solder particles disposed on a carrier
sheet.
BACKGROUND OF THE INVENTION
All modern electronic products including computers, consumer
electronics, telecommunication equipment and automobiles require
circuit interconnection. While off-chip interconnection and
packaging densities have improved over the years, the progress has
been far slower than the improvement in on-chip semiconductor
devices where the dramatic decrease in circuit feature size to the
micron level has increased IC circuit densities from 250K to 64 MB
in memory devices. The typical width of present-day circuit contact
pads for solder interconnection is about 25 mils (625 .mu.m) for
printed circuit boards, and about 4 mils (100 .mu.m) for
silicon-on-silicon flip-chip devices. This enormous imbalance
between the micron-level features of silicon devices and the
hundreds-of-microns dimensions required for contact pads has forced
very inefficient device integration. Around a small semiconductor
element much "real estate" is wasted on fan-outs to larger-area,
soldering contact pads. This fan out also results in longer travel
path for electrons and hence slower device speed than could be
realized with a compact, high-density interconnection scheme.
Most circuit board interconnections between mating contact pads
utilize solder materials, such as the eutectic lead-tin solder
(37Pb-63Sn). The solder materials are melted and solidified either
by wave soldering or by surface mounting techniques. These
techniques are described in "Soldering Handbook for Printed
Circuits and Surface Mounting", by H. H. Manko, Van Nostrand
Reinhold, New York, 1986, which is incorporated herein by
reference. The surface mounting procedure is typically based on
screen printing technology with the wet solder paste printed on
each circuit pads of the substrate board to be solder
interconnected. Alternatively, the solder may be deposited on each
of the contact pads by physical or chemical vapor deposition or by
electrochemical deposition, in combination with
photolithography.
Two of the main technical barriers to the achievement of high or
ultra-high density interconnections using smaller contact pad size
are i) the absence of an industrially-viable technique for screen
printing the solder paste below about 6 mil line width resolution
and ii) the difficulty and high cost of large-area photolithography
below the resolution of about 2 mil. Accordingly, there is a need
for a new high density interconnection technology which is not
restricted by screen printing or lithography. The present invention
discloses such a technology.
SUMMARY OF THE INVENTION
In accordance with the invention, an electronic device having one
or more contact pads is placed in contact with a carrier sheet
bearing an array of transferable solder particles. Heat is applied
to adhere the solder to the contact pads, and solder is selectively
transferred onto the contact pads. In a preferred embodiment the
solder-carrying medium comprises elastomeric material and the
solder particles comprise solder-coated magnetic particles.
Application of a magnetic field while the elastomer is curing
produces a regular array of solder coated particles. Using this
method, devices having smaller than conventional contact structures
can be readily interconnected.
BRIEF DESCRIPTION OF THE DRAWINGS
The nature, advantages and various additional features of the
invention will appear more fully upon consideration of the
illustrative embodiments now to be described in detail in
connection with the accompanying drawings. In the drawings:
FIG. 1 is a block diagram showing the steps in making a high
density connection.
FIG. 2 schematically illustrates an electronic device having
conductive contact pads;
FIG. 3 is a schematic illustration of the device of FIG. 1 placed
in contact with an array of transferable solder particles disposed
on a carrier medium;
FIG. 4 shows the carrier medium being removed after protruding
solder particles adhere to the contact pads.
FIG. 5 is a photomicrograph at .times.25 showing top view of the
solder particles tacked on to the contact pads.
FIG. 6 is a photomicrograph at .times.100 showing a top view of
fine solder particles tacked onto 2 mil wide contact pad
regions.
FIG. 7 is a photomicrograph showing the contact pads covered with
solder as a result of melting of solder particles and preferential
wetting of the contact pad regions.
FIG. 8 illustrates the interconnection of a mating device onto the
electronic device.
FIG. 9 schematically illustrates one method of making a
solder-carrying medium.
FIG. 10 is a photomicrograph of magnetic particles dispersed in a
viscous medium in the absence of magnetic field.
FIG. 11 is a photomicrograph similar to FIG. 10 except that a
magnetic field was applied.
It is to be understood that these drawings are for purposes of
illustrating the concepts of the invention and, except for
graphical illustrations, are not to scale.
DETAILED DESCRIPTION
Referring to the drawings, FIG. 1 is a block diagram showing the
steps in making a high density connection in accordance with the
invention. The first step shown in block A is to provide an
electronic device having one or more contact pads. As shown in FIG.
2, such a device 9 typically comprises a substrate 10 having a
substantially planar surface 11 including a plurality of conductive
contact pads 12 rising above surface 11. The substrate 10 is
typically a semiconductor wafer, an epoxy-based printed circuit
board or a ceramic substrate. Contact pads 12 are typically coated
copper. The pads can be coated with various metallic of polymer
finishes for corrosion resistance and improved wetting of molten
solder. Typical coatings are Au, Sn, solder and imidazole. The
device can have numerous circuit elements (not shown) in the
inter-pad areas 13.
The next step, shown in block B of FIG. 1, is to adhere the contact
pads to an array of solder particles transferably disposed on a
carrier sheet. This step is illustrated in FIG. 3 where device 9 is
contacted by an array of solder particles 20 partially embedded in
a carrier sheet medium 21. The particles are adhered to the device
contact pads by the application of heat. The protrusion of the
solder particles 20 beyond the surface of the carrier medium 21
permits easy wetting of the solder to the metallic device pads 12.
Advantageously, the carrier sheet can be provided with a backing
layer 22, such as paper, for strength and easy handling. The
substrate 10 can be pre-heated (and appropriately fluxed if
necessary) before the carrier sheet is placed over it followed by
application of vertical or sweeping pressure (e.g. by weight or
roller action) to either tack or melt the solder particles 20 onto
the contact pads 12 on the substrate 10. Alternatively, the carrier
sheet can be heated together with the substrate under applied
weight.
The dimension of the solder particles and contact pads are chosen
such that only those solder particles facing the contact pads are
tacked or melted, while the particles in the inter-pad areas 13 do
not get tacked or melted onto the inter-pad areas. The surface of
the inter-pad region is typically covered with non-metallic
insulating materials such as polymer, and hence the solder is not
easily tacked or melted onto it.
The third step shown in block C of FIG. 1 is to selectively
transfer the adhered solder particles to the contact pads. This
step is illustrated in FIG. 4 where, after the solder particles 40
are adhered to the contact pads by tacking or melting, the carrier
material that contained the solder particles is removed by peeling
it away. The solder particles 20 that faced the inter-pad areas 13
remain embedded in the carrier material 21 and are removed together
with the carrier. These solder particles remaining in the carrier
can be easily extracted and recycled if desired.
The solder material can be any material with various desirable
solder characteristics, e.g., appropriate melting point,
solderability (wettability), mechanical, thermal, electrical
properties, manufacturability and reliability. Known materials such
as Pb-Sn solders including the most widely used eutectic 37Pb-63Sn,
Bi--Sn, Sn--Ag, Sn--Sb, may be used, or new solder alloys such as
described in U.S. patent application Ser. No. 08/020508, filed Feb.
22, 1993, and Ser. No. 08/055495, filed Apr. 30, 1993, may be
used.
The solder particle shape is advantageously spherical as it is easy
to reproducibly manufacture spheres of relatively uniform size and
shape and to make the transferable carrier medium with relatively
uniformly protruding particles. Desired size range of the solder
particles is 0.2-200 .mu.m, and preferably 0.5-50 .mu.m. Fluxes
such as RMA ("Rosin-Mildly-Activated") may be used, e.g., by spray
coating on either the transferable solder medium or the substrate
to improve the wetting of the solder onto the contact pad metal
surface.
The carrier material can be made from a number of polymers, papers,
or sheets of organic or inorganic materials. An example is an
elastomer such as RTV 615 manufactured and sold by General Electric
Co. The use of an elastomer is particularly advantages for two
reasons. First the elastomer stretches out easily so that the
solder particles tacked onto the contact pads slip out of the
elastomer easily. Second the compliant nature of the elastomer,
which accommodates some variation in pad height or substrate
warpage, ensures most of the solder particles in the carrier medium
will contact the pads during tacking operation. Alternatively,
instead of elastomer, a tacky (or sticky) tape may also be used to
carry the solder particles. In this case, an additional cleaning
step after the solder tacking or melting may be needed in order to
remove the left-over tacky polymer for desirable solderability
during subsequent reflow interconnection. Suitable solvent to
accomplish this cleaning step could be conveniently incorporated
into the flux that will be applied before the reflow
interconnection.
The support layer 22 can strengthen the carrier for easily
handling, e.g., winding onto or unwinding from a spool. This
support layer can be the same elastomer, plastic tape (e.g.
polyethylene), paper or other sheet material. Advantageously layer
22 has a slightly tacky surface so that it adheres to the solder
carrier sheet but can be peeled away from the solder carrier medium
if desired.
FIG. 5 is a photomicrograph representing a top view of the tacked
solder particles (.about.50% surface coverage with .about.35 .mu.m
diameter 37Pb-63Sn solder embedded in a 12 mil thick RTV carrier,
which is then tacked by heating to 150.degree. C./2 min. with
.about.2 psi weight, using a non-activated, water-white rosin flux)
after the solder carrier sheet is removed. The solder particles are
adherent to the 20 mil wide contact pad (Au-coated Cu-surface). The
absence of the solder particles in the intra-pad region is
evident.
FIG. 6 is a photomicrograph showing a top view of finer solder
particles (.about.50% surface coverage with .about.10 .mu.m size
37Pb-63Sn solder in an RTV carrier processed like the example of
FIG. 5) tacked preferentially on 2 mil wide contact pad regions.
The absence of solder in the inter-pad regions is again
evident.
FIG. 7 is a photomicrograph (top view) showing the contact pads
essentially completely covered with solder as a result of melting
of the 35.mu.m diameter solder particles in FIG. 5 and preferential
wetting of the contact pads. It is evident that the solder wets
only the contact pads, leaving the inter-pad region devoid of
solder.
The fourth step in block D of FIG. 1 is to place the contact pads
of the device 9 in contact with the surfaces of another device
(such as another electronic device) to be connected to them. This
is preferably accomplished, as shown in FIG. 8 in a subsequent
reflow interconnection operation by bringing down the mating device
30 onto the device 9 and melting the solder 31. If desired, the
mating surface of device 9 is coated with an appropriate flux 32.
Any one of a number of known heating methods can be used, e.g.,
oven heating or infrared heating. The thickness of the solder layer
can be increased, if desired, by applying the first three steps of
FIG. 1 as many times as is needed in order to achieve proper solder
joint size and geometry during reflow operation.
There are a number of different ways of preparing the solder
carrying medium. For example, the solder medium can be prepared by
sprinkling of solder powder onto the surface of semi-cured, sticky
carrier medium, or by spray coating the solder powder, spin coating
it, or pressing down the sticky side of the medium onto the powder
to tack solder particles. FIG. 9 schematically illustrates a
sprinkling method using a sieve 40 to control the rate of drop of
loose solder particles 41 onto medium 21. The sieve can be lightly
tapped with desired intensity for optimum particle density on the
carrier medium.
The medium 21, which is preferably a polymer such as an elastomer,
may be used in the uncured state or partly cured to retain the
solder particles placed on it. The viscosity and the surface
tension of the elastomer can be adjusted, e.g., by choosing proper
formula or by changing the exposure time or temperature of curing.
This is to ensure that the solder particles are only partially
embedded in the medium so that the part of the solder particle
surface not coated with the polymer protrudes beyond the medium
surface for easy transfer of solder to contact pad. An alternative
way of ensuring the protrusion is to apply a layer, with
controlled-thickness (preferably less than 1/2 of solder particle
diameter), of uncured medium (not shown) on top of pre-cured
medium, and then placing the solder particles onto the surface.
The composite structure with the typical solder particle burial
depth of .about.1/3 to 2/3 of the diameter is then cured for use as
a transferable-solder medium. Another way is to use a tack tape
with thin layer of tacky polymer or organic coating on the surface
to hold the solder particles until they are transferred to the
contact pads.
In order to minimize undesirable electrical shorting between
adjacent contact pads, e.g., caused by statistically possible
percolation of particles (stringer formation), the area fraction
covered by solder particles in the transferable-solder medium is
preferably restricted, to less than 60% coverage and more
preferably less than 40% coverage. By repeating steps 1-3 of FIG.
1, the desired solder thickness can be built up without causing
undesirable intra-pad shorting.
Preferably the probability of the shorting and the variation in
solder amount is reduced by use magnetic separation. The solder
particles can be magnetic particles coated with solder as by
electroless coating of ferromagnetic particles. For example, if 5
.mu.m thick layer or solder is coated on the surface of 7 .mu.m
diameter iron particles, the solder to iron volume fraction ratio
would be about 4:1. In the presence of vertical magnetic field
which is properly balanced against the surface tension of the
matrix polymer, magnetic particles in a viscous medium can be made
to repel each other and form a regular dispersion as shown in U.S.
Pat. No. 4,737,112, issued to Jin et al. on Apr. 12, 1988, which is
incorporated herein by reference. As shown in comparative
micrographs of FIGS. 10 and 11, the presence of vertical magnetic
field achieves a regular (uniform) dispersion of particles with
very little stringer formation. After magnetic separation, the
elastomer can be cured to produce the carrier medium.
The magnetic core could be any one of a number of ferromagnetic
materials: relatively soft magnetic materials such as Fe, Ni, Co,
Ni--Fe (permalloy), Ni--Zn ferrite, Mn--Zn ferrite, or permanent
magnetic materials such as Fe--Al--Ni--Co (Alnico), Fe--C--Co,
hexaferrites, rare-earth cobalt or Nd--Fe--B type magnets. The soft
magnetic materials are easy to magnetize and hence are preferred.
The metallurgical reaction between the solder material and the
magnetic core material should be minimized so as not to
inadvertently deteriorate the solder behavior and properties.
The inventive transferable-solder medium is also suitable for
area-array interconnections as well as multi-layered three
dimensional interconnection for high circuit density. The
limitations in linewidth resolution encountered in conventional
techniques are not present in the new interconnection methodology
and fine-line, high-density contact pads can be easily and
inexpensively coated.
* * * * *